As Rachel Posner, a call girl, Brosnahan's role was slated for a two-episode arc in the critically acclaimed series starring Kevin Spacey as ruthless politician Francis Underwood.

But then showrunner Beau Willimon thought up a series-changing idea.

Willimon, the writer responsible for adapting BBC's "House of Cards" for American audiences, was going to have Corey Stoll's lost boylike Rep. Peter Russo run for governor of Pennsylvania instead of the character he originally envisioned. Considering Brosnahan's appearances on the show had been as Russo's prostitute, Willimon planned for her character to play a crucial role in Russo's downfall.

"She had done such a fine job those first two episodes that I started exploring what it would mean to bring her character back and fully three dimensionalize her," Willimon said. "Rachel was so fantastic when we brought her back that I just wanted to write for her more and more."

For Highland Park's Brosnahan, 23, every episode she returned for was a surprise, a "dream come true."

"I had no idea where the story was going, and so every episode that came up, something new and exciting was happening," she said during a recent interview. "It was just food. I was hungry for it."

Brosnahan has had an appetite for story since she can remember, and her resume displays a feast of rich roles. She booked her first professional gig, a small part in the Michael Bay-produced horror movie "The Unborn," while she was still at Highland Park High School. She was cast in a main stage production at Steppenwolf as a freshman at New York University, a role that landed her on Tribune critic Chris Jones' 2009 list of up-and-comers to watch. And within the past year, Brosnahan appeared in "Beautiful Creatures," which starred Emma Thompson and Jeremy Irons, filmed guest-starring roles in NBC's "The Blacklist" and ABC's soon-to-be-released "The Black Box" and made her Broadway debut alongside Bobby Cannavale in "The Big Knife."

The second season of "House of Cards" was released in its entirety on Valentine's Day. At the end of Season 1, Posner was the only person who could connect Underwood to Russo's tragic end, the only person outside his inner circle who could bring down the eponymous "house of cards."

Brosnahan's Posner is tough, cunning, untrusting and emotionally worn. The first season revealed that Posner dropped out of high school as a sophomore and strongly implied that her father abused her. Since finding herself tied up in Underwood's plans, she's been pushed around by his chief of staff Doug Stamper, played by Michael Kelly. Season 2 sees Posner push back against Stamper and attempt to create a life of her own.

Speaking over Skype, Brosnahan is composed and mature beyond her years, a modern day version of Shakespeare's Rosalind.

"I think she is incredibly calculated because she has to be," Brosnahan said of her character. "She is not in a position of power, and I think something that the show explores is that all of these characters are fighting for power. They are fighting for agency in their lives and having varying degrees of success. I think she is incredibly powerful, and I think she is underrated. … She is that quiet kind of smart."

A call to acting

Born in Milwaukee, but raised in Highland Park since age 4, Brosnahan points to the books on tape her mother used to play as she fell asleep for giving her an actor's imagination.

"I would listen to these amazing narrators (telling) these stories using all different voices, and I used to imagine what the worlds would look like," she said. "I think there was something about that imagining that made me want to act. I found I could so easily imagine those worlds."

Neither of Brosnahan's parents noted a serious interest in performance until she was in Northwood Junior High School's musicals, they said during a phone interview.

While they were supportive, they didn't exactly encourage her to focus on acting, they said. As a teen, she saved her baby-sitting money to attend Broadway Bootcamp, an acting camp.

"She is very self-motivated and always has been," her mother Carol Brosnahan said. "She wanted to teach herself from the beginning. She did it all on her own, and we were really just in the background."

When Brosnahan was 16, she took a class with Carole Dibo, director of Wilmette's Actors Training Center and now Brosnahan's manager. Brosnahan's dedication to the craft was apparent from their first meeting, Dibo said.

Rachel had "a maturity and a commitment to the process of acting that very few kids have at 16," Dibo said. There was just something ingrained and organic about Rachel. She approached her work in a very no-nonsense way and without fear."

In 2009, Brosnahan booked a role in Steppenwolf's "Up," playing a pregnant teenager opposite Martha Lavey. (That spring Brosnahan was also up for a role on a CBS pilot but turned it down for the chance to act with Steppenwolf.) Jones called Brosnahan's performance "guileless" and "long on charm and honesty and short on cutesy contrivance."

"She was capable of improvisation, which is to say she was emotionally available, and she was spontaneous even though she was very disciplined," Lavey said. "She knows how to interact with the person onstage, and emotionally she has a really deep well."

Playing her cards

On "House of Cards," Brosnahan taps that deep emotional well, playing a woman who is simultaneously strong and fragile, secure and vulnerable. It was her ability to act out those dichotomous states that made Willimon want to cast her, he said.

"She is so young and beautiful, and on the surface could seem innocent," he said, "but you know that there is something tough underneath her skin, that she is a fighter, that she has fire in her and there is an element of danger in her eyes that not a lot of people have."

Brosnahan's Posner interacts mainly with Kelly's Stamper. Their relationship is, to put it simply, complex; it's concurrently loving, difficult and dangerous. It's not a stretch to think that Stamper is the reason Posner hasn't been "hurt" or "discarded" by Underwood.

"Rachel has the ability to connect, to sort of lock in with you," Kelly said. "You can get lost in that; you can really engage in a scene with someone when they have that kind of talent."

For Brosnahan, one of the most exciting parts of playing Posner isn't what's written down; it's what's between the lines.

"There were so many levels that could be (explored) with this character, it was fascinating to me," she said. "In Season 1, there is a big chunk where she disappears from Episode 2 to Episode 7 and then she shows up again with a black eye, and you never find out what happened, and so that exploration of what motivates her all the time and how her motivations change has been fascinating to me."

A few days before the Season 2 premiere of "House of Cards," Brosnahan sent her mother a text message that said, "Thought you two might appreciate this." Attached was a photo of a Times Square billboard promoting the show's second season and featuring a larger-than-life image of Brosnahan's face.

"I sent her a text message back, and I said, 'From that little girl in middle school who wanted to be an actress all the way to a billboard in Times Square. Wow,'" Carol Brosnahan said.

Ask the people who've worked with Brosnahan, and there is an overwhelming feeling that Brosnahan is at the tipping point of her career, a sentiment that it's only up from here.

"They say that some people have it, and she just has it," Kelly said. "She just has that thing, that magnetism on screen. She has the ability to be so natural in front of the camera and so understated and yet so intriguing that you just want to watch her."